For cellular communications, especially in the uplink (UL), it is important to maintain a low Peak-to-Average-Power Ratio/Cubic Metric (PAPR/CM) of the transmitted signal for less power backoff in the amplifier. This in turn offers better coverage and less power consumption of the user device. Multi-carrier signals, such as Orthogonal Frequency Division Multiplexing (OFDM), are known to exhibit rather large PAPR/CM. One method used in Long Term Evolution (LTE) uplink for reducing the PAPR/CM is to apply a Discrete Fourier Transform (DFT) precoder, which applied prior to the Inverse Fast Fourier Transform (IFFT) operation, produces a Single Carrier—Frequency Division Multiple Access (SC-FDMA) signal.
In LTE uplink, the control channel, Physical Uplink Control Channel (PUCCH), is conveying Uplink Control information (UCI), e.g., Hybrid Automatic Repeat reQuest-ACKnowledgement (HARQ-ACK), Channel State Information (CSI) and Scheduling Request (SR), by using different PUCCH formats. The PUCCH formats are distinguished by their payload capacities as well as other properties, e.g., some are based on modulated sequences while some are DFT precoded and some are capable of Code Division Multiplexing (CDM) among User Equipments (UEs). PUCCH formats 1/1a/1b/2/2a/2b/3/5 are transmitted in one Physical Resource Block (PRB) pair (i.e., one PRB in the first slot and one PRB in the second slot of a subframe), and PUCCH format 4 is transmitted with one, or more than one, PRB pair(s) per slot. For PUCCH format 4, the transmission is made in one cluster of one or multiple PRBs contiguously located in the frequency domain within a slot. This assures low PAPR/CM. Furthermore, for all PUCCH formats, frequency hopping is used between the two slots in a subframe for the PRB pairs, such that the PRBs are located towards different edges of the carrier in the two slots.
LTE can also be deployed for transmissions in unlicensed spectrum with carrier aggregation, i.e., utilizing Licensed Assisted Access (LAA) where an unlicensed carrier is operated as a Secondary cell (SCell) in conjunction with a Primary cell (PCell) located in licensed spectrum. For LAA, a first regulatory requirement is that the occupied channel bandwidth shall be between 80% and 100% of the declared nominal channel bandwidth. The occupied channel bandwidth is the bandwidth containing 99% of the power of the signal. This requirement does not mandate that only a single UE can occupy 80-100% of the carrier bandwidth. For example, it would be possible to multiplex PUCCHs from several UEs in an UL subframe over the whole carrier bandwidth using some form of interleaved Frequency Division Multiplex (FDM) allocation, while fulfilling the occupied channel bandwidth requirement. In addition, a second regulatory requirement is the maximum transmission power in a narrow band. For example, in the frequency band 5250-5350 MHz, transmissions shall be limited to a maximum mean Equivalent Isotropically Radiated Power (EIRP) density of 10 mW/MHz in any 1 MHz band. This implies that, in order not to limit the transmit power, it is beneficial to allocate the resources in as many ‘1 MHz’ bands as possible.
A PUCCH being transmitted in frequency localized manner may not fulfill the bandwidth occupancy requirement, since it may not span a sufficient bandwidth in any given slot. In order to efficiently meet the bandwidth requirement, extending the current single cluster allocation to allow multi-cluster (>2) allocation (e.g., PRBs spaced uniformly in frequency) has been identified as a candidate waveform for PUCCH. The LTE PUCCH formats, e.g., PUCCH formats 1/1a/1b/2/2a/2b/3/5, occupy only one PRB pair in the frequency domain, which does not fit into the multi-cluster PUCCH resource with multiple PRB pairs. It would still be desirable if the multi-cluster PUCCH could be based on existing LTE PUCCH formats, e.g., PUCCH formats 3/4/5, which can carry sufficiently large payloads while benefitting from low PAPR/CM due to DFT preceding. This would be advantageous since much of the receiver and transmitter processing of the PUCCH could be reused. Furthermore, the multi-cluster PUCCH should preferably be designed for low PAPR/CM to avoid power backoff which could imply coverage limitation.